With never-ending changes and improvement of the technologic development, the manufacturing technologies of visual-audio compression and laser electro-optical products become more matured than ever before. Optical storage media have developed toward the objectives of smaller size and larger storage capacity step by step. And, since the advantages of optical disks have larger in storage capacity, lower in cost, easier to saving, longer in storage period and uneasy in information damaged and so on, the optical disks have been one of essential saving medias already in our modern society.
In 1980, Philips Company proposed a recording medium having a transparent substrate and a plurality of data pits, and accessed by irradiating a laser beam through the transparent substrate. To meet the needs of a variety of data storage applications, various types of compact disk (CD), for example, CD, CD-G; CD-I, photo-CD, VCD, CD-R and CD-RW, have been suggested and realized. In 1995, due to the increasing demand for larger amount of information, a high storage capacity optical disk, digital versatile disk (DVD), is proposed to provide 4.7 GB data on a single layer disk with a diameter of 12 cm. Next, a new generation of Blu-ray disk also published in 2002 by SONY. Recently, the largest storage capacity of Blu-ray disk with a single layer has achieved to 25 GB.
On the other hand, with the popularity of 3D films, 3D televisions and fiber optic networks, the requirements of a better quality of videos and of a larger storage capacity of saving media are increased. In the past, the following methods were used to increase the storage capacity of the conventional optical disks: (1) a more efficient coding and decoding technique, (2) a small size of all the pits and their pitches of the tracks on optical disks, (3) using the shorter wavelength of a light source, (4) increase of the numerical aperture value of the objective lens, (5) using a volumetric technology such as multi-layer recording, holography, etc. However, aforementioned methods are only the optimizations under the diffraction limit. Basically, the access and write data pits of the optical disk are still restricted by diffraction limit. Therefore, it is difficult to achieve a major breakthrough to increase the storage capacity of the optical disks in the prior art.